The present invention relates to the general field of fuel metering units with variable geometrical aperture. It finds a preferential application to fuel metering units fitting out fuel supply circuits of an aircraft engine, and more particularly to circuits delivering fuel for feeding combustion injectors of the engine.
Generally, a fuel supply circuit of an aircraft engine comprises a pumping system consisting of a low pressure pump connected upstream to the fuel tank of the aircraft and associated downstream with a high pressure pump which is driven by the engine via an accessories gear box (or AGB). Downstream from the high pressure pump, the fuel supply circuit is separated into two branches, one of the branches comprising a bypass control valve and a fuel metering unit allowing regulation of the fuel flow rate feeding the combustion injectors of the engine, and the other branch comprising a heat exchanger and sending the fuel towards actuators of variable geometries of the engine (for example, air discharge valves or control valves for vanes of variable pitch). The excess fuel flow rate generated by the high pressure pump is returned to the inlet of this pump through the bypass control valve, which partially contributes to heating up the circuit, particularly at low flow rates of the engine where the recycling rate is maximal.
Such a fuel supply circuit should be protected against the risk of icing of the water contained in the fuel. Indeed, the pressure and the temperature encountered during flight may cause water to pass from the liquid state to the solid state and cause malfunctions of the fuel supply circuit.
In conventional architectures of fuel supply circuits, the units which are the most sensitive to icing are supplied with heated fuel at a positive temperature for a heat exchanger. However, when the oil is used for heating the fuel, the heat power available in the oil system of the engine only gives the possibility of sufficiently heating, from a dedicated oil/fuel exchanger, the fuel for the low flows sent towards the actuators of variable geometries of the engine. Indeed, this heat power is not sufficient for heating the fuel for the large flows crossing the fuel meter, for example during the take-off and ascent phases of the aircraft. Now, the units which are the bypass control valve, the fuel meter and the combustion injectors of the engine may be sensitive to icing of the fuel.
In order to make the bypass control valve less sensitive to icing of the fuel, resorting to a server-controlled valve with a large force margin and with a heated pressure difference detector at the terminals of the valve is known. As for the combustion injectors, they may generally operate for a significant time under icing conditions before leading to malfunction of the engine. This time is generally longer than the time of the mission portion where the fuel is found under icing conditions.
On the other hand, the fuel metering unit is a unit which is made insensitive to icing of the fuel only with difficulty, in particular when this is a fuel meter provided with a metering slot with an exponential passage profile. Such a metering unit has the advantage of giving the possibility of obtaining high accuracy in the metering of fuel at low flow rates. However, when it is supplied with fuel under icing conditions, the small width portion of the metering slot tends to be partly blocked because of ice particles which will agglomerate therein. More specifically, this partial blocking typically appears for high flow rates (for which the fuel is at a negative temperature) in the narrow portion of the slot. The result of this is that in the case of returning to low flow rates after operating with a high flow rate (typically in the case of returning to an idling phase of the engine), there exists a non-negligible risk that the injected fuel flow rate becomes less than the limit set by the extinction abutment, and therefore there is a risk of extinction of the combustion chamber of the engine.
The published patent application FR 2825120 A1 discloses a fuel metering unit provided with a metering slot with an exponential passage profile, the metering unit being of cylindrical geometry with a movable element as a piston. The metering slot is split into two portions spaced apart from each other circumferentially and separated by sealing means, so as to produce an outlet for low flow rates separated from the outlet for high flow rates. The outlet dedicated to low flow rates corresponds to the narrow portion of the slot, while the outlet dedicated to high flow rates corresponds to the widest portion of the slot. Such a device is particularly suitable for limiting over-speed by only obturating the outlet dedicated to high flow rates.
However, this known device does not give the possibility, when it is supplied with fuel under icing conditions, of preventing the output dedicated to low flow rates, i.e. the small width portion of the slot, of being partly blocked because of ice particles which will agglomerate therein.